The present invention relates a wire printing head for a high resolution printing machine and the method for its manufacturing, wherein the printing machine comprises a printing platen for a recording medium and a carriage transversely movable with respect to the printing platen and for mounting the printing head, and wherein the head comprises a guide matrix adjacent to the platen and a plurality of wires having a given nominal section, the wires being guided by holes in the guide matrix and longitudinally movable to define a corresponding plurality of printing dots on the recording medium.
U.S. Pat. No. 5,054,942 discloses a printing head of the above mentioned type, comprising twenty four wires of a diameter of 0.25 mm. This head can be used in a printer forming a dot matrix having a vertical resolution of 1/180 to enable printing of characters similar to characters printed from a printing element using a continuous but heavy profile.
Dot characters having a similar look to characters which have a continuous and thin profile require wires having a diameter of 0.20 mm or less. The printing wires are, on the other hand, subject to relevant stresses either internally and externally of the head. The use of wires with reduced diameter is therefore limited by the fact that the thin wires can be subject to permanent deformations. A first cause of deformation, for combined bending and compressive stress, is determined by the printing stresses which arise on the extremities of the wires at the time of the printing. However, the main cause of deformation is determined by over-stressing by bending to which the printing extremities of the wires which project from the guide matrix can be subject. This occurs, in particular conditions, during movement of the carriage and the printing of the wires, when the same extremities meet with obstacles, for instance sharp variations in the thickness of the recording medium.
There are in commerce printing wires, which provide strokes of different sections along their length, decreasing towards the printing zone. Therefore the guide supports, within the head, require guide holes having a larger diameter than the diameter of the holes of the guide matrix. These wires avoid the problem of deformation for the combined bending and compressive stress but do not avoid the deformations of the terminal parts of reduced section which project from the guide matrix, in the case of over-stress by bending.